The main aim of gasification is to convert the chemical energy embedded in a solid fuel into the chemical energy of a resulting gaseous fuel. The conversion efficiency is not 100%, mainly because gasification has to be carried out at an elevated temperature. The raw feed material and the gasifying agents must be heated up to elevated temperature, which are endothermic processes. The actual conversion of a solid fuel into a gaseous fuel may be exothermic or endothermic, depending largely on the oxygen consumption. After gasification, the gaseous fuel must be cooled down, which is an exothermic process. Some of the sensible heat and latent heat embedded in the gaseous fuel cannot be entirely recovered as useful energy. Therefore, the mismatch (gap) between the sum of the (endothermic) heat demand for heating up the reactants and for gasifying the solid fuel and the exothermic heat released by cooling down the gaseous fuel is a major source of inefficiency. For a gasification process to be operated at a commercial scale, burning part of the fuel with air (oxygen) is the common practice to meet this energy gap.
There are two major types of strategies to improve the efficiency of the gasification process. On one hand, the gasification temperature may be reduced with concurrent minimization of oxygen consumption. This particular strategy is generally limited by the reaction kinetics of the gasification process as determined by the composition of the carbonaceous material, especially the gasification of char, as well as the presence of tar residue in the gaseous fuel. For the gasification of low-rank fuels such as biomass, a major limiting factor to achieving fast gasification rates is the adverse effects of volatile-char interactions.
The second strategy to improve the gasification efficiency is to recuperate the thermal energy in the product gas into the chemical energy of the gaseous fuel. The recuperation of thermal energy is a process to increase energy. In the operation of a commercial gasifier, this means finding ways to heat up the fuels and gasifying agents and/or to meet the energy demands of endothermic pyrolysis/gasification/reforming reactions by using the sensible/latent heat of the hot product gas stream. Low-rank fuels such as biomass have very high reactivity and can be gasified at a much lower temperature than that of a high-rank fuel. Therefore, the gasification of low-rank fuels offers an excellent opportunity to recuperate low-temperature (i.e. low grade) heat into the chemical energy of the gasification fuel gas.
There are three categories of pyrolysers, based on the mode of heat supply.
The first category is to use a heat carrier that is physically mixed with the feed material. Conventionally, a fluidized-bed pyrolyser employs direct heat supply by physical mixing. An inert hot gas stream may be used as the heat carrier and is rapidly mixed with a feed material, such as biomass, (and optionally sand) to pyrolyse the feed material. These types of pyrolysers are not very suitable for recuperating the thermal energy in the gasification product gas because the gasification product gas would be excessively diluted by the inert gas, leading to very low heating values of the gasification product gas and subsequent difficulties for its combustion in the downstream gas engine for electricity generation (or combined heat and power generation, CHP).
Some pyrolysers are configured for direct chemical heating, whereby heat for drying biomass, heating of biomass or pyrolysis, or part of it, is performed by exothermic reactions between the feed material (or pyrolysis products) and oxygen (air). The most important advantage is that large amounts of heat can be supplied rapidly. When cold feed material is fed into a gasifier directly, a significant fraction of the thermal energy to dry, heat up and pyrolyse the feed material may be supplied in this way. For a standalone pyrolyser, however, care must be taken to carefully manage the associated safety issues to avoid the presence of possible explosive mixtures in a cold region.
The third category of pyrolysers is configured to employ indirect heat supply via a heat exchanger. Typically, such pyrolysers take the form of a screw (auger) pyrolyser which is heated externally. While this category of pyrolyser may be suitable, in principle, for recuperating the thermal energy in the product gas into the chemical energy of the gaseous fuel, as outlined above, currently available pyrolysers suffer from a limited amount of heat exchange area.
In addition for pyrolysis to be part of gasification (or even combustion), pyrolysis is also a route of processing solid fuels, e.g. to produce bio-oil, biogas and biochar. In practical operations, minimizing the use of an inert carrier gas or solid is important for maximizing the overall process efficiency and economy. Indirect heating offers significant benefits; however, the provision of abundant heat transfer surface area remains a technical challenge.
Bio-oil is a pyrolysis product with exceedingly complicated composition. For improving the efficiency of bio-oil upgrading (biorefinery), it would be beneficial if bio-oil components released at the early stages of pyrolysis can be separated from those released at the later stages.
Solid handling and transferring are a significant task in carrying out the pyrolysis of solid fuels. A feeder is normally needed to add the feedstock into the pyrolyser. Additional mechanisms are also required to transfer the pyrolysing solid feedstock and products across the pyrolyser. Integrating the various solid handling and transferring mechanisms would result in improved process efficiency and economy.
There is therefore a need for technological advancement.
Any references to background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the apparatus as disclosed herein.